Aluminum silicon (Al—Si) eutectic alloys, which have a slightly lower melting point than aluminum-based materials, have been mainly used as materials for brazing aluminum materials or aluminum alloy materials (hereinafter simply referred to as aluminum-based materials).
For good adhesion between a brazing material and an aluminum-based material, removal of oxide film, etc., present on the surface of the aluminum-based material is required. To remove such an oxide film, etc., it is necessary to supply a flux together with a brazing material to the portion to be brazed.
In recent years, non-corrosive or water-insoluble fluoride-based fluxes are mainly used in place of conventional chlorine-based fluxes.
Such fluoride-based fluxes have excellent features, such as almost no corrosivity, water solubility, or moisture absorbency compared to conventional chlorine-based fluxes.
Since containing magnesium increases the strength of aluminum alloys, it is generally known that aluminum alloy products such as heat exchangers, automobile parts, bicycle parts, and pipes are preferably produced by brazing aluminum alloy materials containing 1.5 wt % or more of magnesium that has high strength and excellent corrosion resistance. As a result, the thickness of the product is reduced, thus reducing the weight.
However, in brazing using a fluoride-based flux, when the magnesium content in an aluminum alloy is 1.5 wt % or more, magnesium is reacted with the fluoride-based flux to produce a high-melting-point compound on the aluminum alloy surface, which reduces the effect of the flux and inhibits wetting and permeation of a brazing material. Thus, the brazing properties are remarkably decreased, making it impossible to perform brazing sometimes.
Patent Literature 1 discloses “a flux composition for brazing Al materials, the composition mainly comprising a molten and coagulated product of a powdery mixture of Lif, AlF3, and CsF, wherein the composition ratio of the three components is adjusted to within the range enclosed by four lines, i.e., line A connecting (2, 0, 98) and (2, 98, 0), line B connecting (0, 22, 78) and (78, 22, 0), line C connecting (30, 0, 70) and (30, 70, 0), and line D connecting (60, 40, 0) and (0, 70, 30) in the triangular coordinates indicating LiF mol %, AlF3 mol %, and CsF mol %. Patent Literature 1 uses a Zn—Al solder (melt starting temperature: about 380° C. or more) as a brazing material and has a problem of pitting corrosion due to the difference in corrosion potential.
Patent Literature 2 discloses a flux-containing Al alloy brazing material comprising a flux component, and Al, Si, and Cu as a brazing material component, wherein the Si content and the Cu content in the brazing material component are respectively 0.05 to 20 wt % and 5 to 10 wt %; the flux component contains KF, LiF, and CsF, and is a fluoride-based flux in which KF, LiF, and CsF are mixed in the range enclosed by a liquidus line at 550° C. in the ternary phase diagram of KF—LiF—CsF; the brazing material component and the flux component are added in a weight ratio of 99.9:0.1 to 70:30; and the flux-containing Al alloy brazing material has a density of 90% or more of the theoretical value. Patent Literature 2 discloses that an Al—Si—Cu alloy (melt starting temperature: about 524° C. or more) used as a brazing material can braze an A5052 alloy containing 2.2 to 2.8 wt % of magnesium; however, an Al—Si eutectic alloy (Si content: 7 to 12 wt %, A4343 alloy and A4047 alloy, melt starting temperature: about 577 to 615° C.) used as a brazing material cannot braze the A5052 alloy.
Patent Literature 3 discloses a flux for brazing an aluminum-based material, comprising, expressed in mol %, potassium fluoride in an amount exceeding 0 mol % to less than 42 mol %, cesium fluoride in an amount of 2 mol % to less than 74 mol %, and aluminum fluoride in an amount of 26 to 67 mol %, or potassium fluoride in an amount of 42 to 54 mol %, cesium fluoride in an amount exceeding 20 to 32 mol %, and aluminum fluoride in an amount of 26 mol % to less than 38 mol %, with potassium and cesium present as fluoroaluminate, and free cesium fluoride and potassium fluoride not included. In Patent Literature 3, when an Al—Si eutectic alloy (Si content: 7 to 12 wt %, A4343 alloy, A4047 alloy, melt starting temperature: about 577 to about 615° C.) is used as a brazing material, the limitation of the magnesium content is 1 wt % (A6061 alloy), and an A5052 alloy containing 2.2 to 2.8 wt % of magnesium cannot be brazed.